1. Field of the Invention
The present invention relates to a digital servo system and more particularly, to a digital servo system in which a servo system for controlling the phase and the speed of a rotary body such as a cylinder motor, a capstan motor in a video tape recorder (referred to as VTR hereinafter) is achieved using a microcomputer.
2. Description of the Prior Art
Conventionally, in a VTR, for example, in a two head helical scanning type VTR, there have been provided a cylinder motor for driving rotation of a rotary head and a capstan motor for driving traveling of a tape. At the time of operation of the VTR, the rotational phases and the rotational speeds of the above described cylinder motor and the capstan motor serving as driving means are servo controlled, so as to correctly control the speed and the phase of rotation of the rotary head and the phase and the speed of traveling of the tape.
More specifically, at the time of recording in the VTR, the rotational speeds of the cylinder motor and the capstan motor are controlled such that the rotational speeds of both the motors take a predetermined value, and the rotational phase of the cylinder motor is controlled such that the rotational phase of the rotary head and the phase of a vertical synchronizing signal in a video signal to be recorded have a predetermined phase relation. In addition, the rotational phase of the capstan motor is controlled such that the rotational speed of the capstan motor is held at the above described predetermined value with accuracy.
On the other hand, at the time of reproduction in the VTR, the rotational speeds of the cylinder motor and the capstan motor are controlled such that the rotational speeds of both the motors take a predetermined value, and the rotational phase of the cylinder motor is controlled such that the rotational phase of the rotary head and the phase of a predetermined reference signal have a predetermined phase relation. In addition, the rotational phase of the capstan motor is also controlled for correct tracking.
A servo control system for the above described control is divided into an analogue system and a digital system. The analogue servo system has a simple circuit structure. However, the system is liable to be affected by, for example, the change of a power-supply voltage, the change of temperature and the change with time, so that stable operation cannot be ensured.
On the other hand, in a digital servo system comprising a counter and the like and utilizing a clock signal, the above described disadvantages are eliminated. In particular, since considerable progress has been made in the digital integrated circuit technique, such a digital servo system is utilized more often. As an example, a digital servo system using a microcomputer is disclosed in, for example, U.S. Pat. Nos. 4,584,507 and 4,668,900.
FIG. 1 is a schematic block diagram showing a part of a digital servo system for a cylinder motor, which comprises an IC (LC7415) developed for such a digital servo control. Referring to FIG. 1, an IC 1 comprises a circuit 2 responsive to a detection signal from a cylinder motor (not shown) for generating a phase error signal of the cylinder motor, a circuit 3 also responsive to a detection signal for generating a speed error signal, D/A converters 4 and 5 and amplifiers 6 and 7. The phase error signal generated in the circuit 2 is converted into an analogue signal by the D/A converter 4 and the analogue signal is amplified by the amplifier 6 and then, outputted from the IC 1 to the exterior. In addition, the speed error signal generated in the circuit 3 is converted into an analogue signal by the D/A converter 5 and the analogue signal is amplified by the amplifier 7 and then, outputted from the IC 1 to the exterior. The analogue phase error signal and the analogue speed error signal outputted from the IC 1 are added to each other outside the IC 1 and the added signal is suitably amplified by an amplifier 8 and then, applied to a cylinder motor driving circuit (not shown) as a servo control signal. Such analogue addition of error signals performed outside a microcomputer is disclosed in an article by M. Endo et al., entitled "VTR Control Circuit", SANYO TECHNICAL REVIEW, VOL. 17, NO. 2, August 1985, pp. 45-50, Japanese Patent Laying-Open Gazette No. 190744/1986 and U.S. Pat. No. 4,536,806.
However, the digital servo system comprising the D/A converters 4 and 5 inside the IC 1 and performing analogue addition of the phase error signal and the speed error signal outside the IC 1 presents the following problems.
FIG. 2 is a diagram for explaining schematically the principle of, for example, generation of the phase error signal of the cylinder motor in the VTR. FIG. 2(a) shows a signal indicating the rotational phase of the cylinder motor actually detected and more particularly, a signal obtained by, for example, frequency-dividing 24 FG Frequency Generator) pulses generated per one rotation of the cylinder motor into 1/2. In FIG. 2(a), a waveform represented by a solid line shows a signal obtained by frequency-dividing into 1/2 the FG signal generated when the cylinder motor is rotated in a predetermined correct phase relation. In addition, a dotted line shows a case in which the rotational phase of the cylinder motor is slightly advanced from the correct rotational phase (represented by the solid line).
On the other hand, FIG. 2 (b) is a diagram for explaining the relation between the change of the rotational phase of the cylinder motor and the amplitude of the phase error signal generated in response to the change. The minimum voltage value and the maximum voltage value which the phase error signal supplied to a motor driving system can actually take are 0 V and a predetermined value (for example, 5 V), respectively. In addition, in the digital servo system, the amplitude of the phase error signal is represented by the number n of bits of the digital phase error signal, "0" corresponding to the above described minimum voltage value (0 V) and "2.sup.n -1" corresponding to the above described maximum voltage value (5 V). Furthermore, in FIG. 2(b), a period "T.sub.DP " when the amplitude takes the minimum value 0 is referred to as a "bias period" and a period "T.sub.SP " when the amplitude changes from the minimum value 0 to the maximum value "2.sup.n -1" is referred to a "clock range".
As can be seen from FIG. 2, if the cylinder motor is correctly rotated in a predetermined phase relation, the amplitude of the phase error signal is fixed at an almost intermediate point A between the minimum value 0 and the maximum value (2.sup.n -1), so that servo control is performed in response to the phase error signal having an amplitude of about (2.sup.n -1)/2. The servo control allows the above described predetermined phase relation to be maintained. This intermediate point A is referred to as a lock point hereinafter.
However, as represented by the dotted line in FIG. 2(a), when the rotational phase of the cylinder motor begins to be shifted in, for example, an advanced direction, the amplitude of the phase error signal is decreased from the above described lock point A to a point B in response to the shift. More specifically, a servo signal supplied to the cylinder motor driving system is decreased and force for restraining rotation of the motor is applied, so that the advanced phase (represented by the dotted line) is returned to a predetermined phase relation (represented by the solid line). Contrary to this, when the rotational phase of the cylinder motor is delayed from the predetermined phase relation, it is clear that the amplitude of the phase error signal is increased from the lock point A in response to the delay. Consequently, the servo signal supplied to the cylinder motor driving system is increased and force for increasing rotation of the motor is applied, so that the delayed phase is returned to the predetermined phase relation.
On the other hand, when the rotational phase of the motor is significantly advanced so that the phase shift goes out of the lock range T.sub.SP and comes within the bias period T.sub.DP, the phase error signal becomes 0, whereby a digital servo is not operated. More specifically, once the rotational phase of the motor goes out of the lock range, active force for capturing the phase error signal up to the lock point A is not applied, so that the function of the servo system is stopped until the rotational phase of the motor is naturally delayed so that the phase shift comes within the lock range T.sub.SP.
The range of the lock range T.sub.SP a problem. The range of the lock range T.sub.SP is determined by the number n of bits of the phase error signal. More specifically, the range is determined absolutely by the period of a clock signal which defines the minimum resolution in the direction of the time base and the number n of output bits. For example when the period of the clock signal is 1 .mu.sec. and the number n of bits equals 10, the lock range T.sub.SP is 1 .mu.sec..times.(2.sup.10 31 1)=1023 .mu.sec. In consideration of the resolution in the direction of the time base, the frequency of the clock signal cannot be decreased, that is, the period thereof cannot be increased. Thus, in order to increase the lock range T.sub.SP, the number n of output bits must be increased. More specifically, if a constant amplitude (for example, 5 V) of the phase error signal is predetermined in the digital servo system, the increment of the amplitude per one clock period is decreased when the number n of bits of the phase error signal is increased. As represented by the dotted line in FIG. 2(b), a value (referred to as conversion gain hereinafter) indicating inclination of the slope in the lock range, that is, the magnitude of the error signal relative to the change of the rotational phase is decreased, so that the lock range T.sub.SP is increased. When the lock range T.sub.SP is increased, the range in which servo operation can be performed relative to the phase shift, that is, the range (referred to as capture range hereinafter) indicating what phase shift is captured up to the above described lock point is increased. On the other hand, if the number of bits of the phase signal is small, the lock range T.sub.SP is decreased, so that it is clear that the above described capture range is decreased. Furthermore, the foregoing description is also applied to control of the rotational speed of the cylinder motor.
Returning to the description of the digital servo system shown in FIG. 1, the error signals generated in the circuits 2 and 3 are converted into analogue signals by the D/A converters 4 and 5, respectively, and then the signals are added to each other. Thus, the number of bits of each of the error signals is limited to the number of bits which can be converted by the D/A converters 4 and 5. For example, when a D/A converter of an R-2R type is employed, cost is increased if the number of bits is increased. When a D/A converter by pulse width modulation (PWM) is employed, the period of the output signal is increased if the number of bits is increased. Consequently, the time constant of a filter for smoothing becomes large, so that servo control is liable to be affected. More specifically, in the conventional digital servo system for performing analogue addition of the error signals outside the IC as shown in FIG. 1, since the number n of bits of the error signal cannot be increased, the lock range T.sub.SP cannot be increased, so that conversion gain thereof is increased. As a result, the error signal is considerably changed by a slight phase shift, so that servo control is released. More specifically, in the conventional digital servo system, the capture range of the digital servo system is decreased, so that the motor can not be correctly servo controlled. In contrast to the conventional digital servo system for performing analogue addition of the error signals, a digital servo system for adding in a digital manner a speed error signal and a tracking error signal within a microcomputer is proposed, which is disclosed in, for example, Japanese Patent Laying-Open Gazettes Nos. 162855/1986 and 172245/1986. However, these systems fail to describe the above described problem of conversion gain of the error signals.
Meanwhile, in the servo system for the cylinder motor in the two head helical scanning type VTR, at the time of reproduction, the rotational phase of the cylinder motor is controlled such that the rotational phase of the rotary head is synchronized with the applied reference signal as described above. On the other hand, at the time of recording, the rotational phase of the cylinder motor is controlled such that the rotational phase of the rotary head and the vertical synchronizing signal in the video signal to be recorded have a predetermined phase relation. An example of such phase control is disclosed in, for example, Japanese Patent Laying-Open Gazette No. 136090/1981. The predetermined phase relation is generally determined by a standard. According to a standard of an NTSC (National Television System Committee) system concerning 8 mm VTR, control must be performed such that the phase difference between an edge of a head switching signal (RFSW) associated with the rotational phase of a head and the vertical synchronizing signal in the video signal to be recorded is 6H.+-.1.5H (H:one horizontal scanning period). Such a phase difference is generally determined within every VTR. In particular, an apparatus for automatically adjusting such a phase difference is proposed, which is disclosed in, for example, Japanese Patent Publication No. 4449/1977.
However, since such an automatic phase adjusting apparatus is adapted such that the phases of a reference signal whose phase is adjusted to coincide with a particular phase of a composite synchronizing signal and a rotary pulse obtained from the cylinder motor are compared with each other, the structure is very complicated.
On the other hand, in the digital servo system, in order to improve performance of a rotational phase servo system of the motor, the sampling frequency of servo control must be set high. This is because if the sampling frequency is low, it becomes difficult to perform servo control quickly in response to a disturbance which may be caused. More specifically, in order to increase the sampling frequency of the digital servo control, an internal phase reference signal having a higher frequency than that of the vertical synchronizing signal in the video signal to be recorded (having a period of one-i-th (i:an integer)) and synchronized with the vertical synchronizing signal must be generated so that servo control is performed in response to the internal reference signal. In the digital servo system, the clock signal which provides a basis for operation of the system is generally generated by utilizing the frequency of a color subcarrier of the video signal to be recorded. However, in particular, if and when it is desired to achieve the digital servo system using a microcomputer, the color subcarrier having a high frequency of the video signal may not be utilized as it is, because the clock frequency of the microcomputer has a predetermined upper limit. In the digital servo system using the microcomputer, a phase reference signal synchronized with the vertical synchronizing signal and having a period of one-i-th must be generated as an internal reference signal for servo control, irrespective of the frequency of the color subcarrier of the video signal.
Additionally, the VTR comprises several kinds of modes of special reproduction such as still reproduction, slow reproduction and high-speed reproduction, in addition to a normal reproduction mode. In the special reproduction modes, the relative speed between the rotary head and a magnetic tape is different from the relative speed at the time of recording. Consequently, in the special reproduction modes, control is achieved such that the rotational speed of the rotary head is slightly changed depending on the modes. Such control is disclosed in Japanese Utility Model Publication No. 6905/1985 .
Meanwhile, in order to change the rotational speed of the rotary head as described above, a constant of a rotational speed control system of the cylinder motor in the digital servo system, that is, a speed bias period and the frequency of the phase reference signal must be changed. However, even if the constant and the frequency are rapidly changed, the number of rotations of the cylinder motor cannot be rapidly changed, so that a phase servo for the cylinder motor is unlocked until the cylinder motor attains a predetermined rotational speed after the mode is changed. When the rotary head attains a predetermined rotational speed and again enters a phase locked state, the rotational speed of the cylinder motor may be temporarily changed considerably by the phase error signal supplied to the cylinder motor driving system. Such a large change of the rotational speed of the cylinder motor causes rolling of a reproduced image and release of color synchronization. Consequently, a digital servo system is required in which the rotational speed of the cylinder motor can be changed with the rotational phase being always locked.
On the other hand, in a special state, various characteristics may be improved if servo control of the rotational phase of the cylinder motor is released. For example, in the helical scanning type VTR, only the rotational speed of the cylinder motor is controlled and control of the rotational phase is released in an intermittent slow reproduction mode in which the tape is intermittently moved so that still reproduction and normal reproduction are alternately repeated, for the following reason. More specifically, since in the still reproduction and the normal reproduction, the relative speeds between the rotary head and the magnetic tape are changed, so that the periods of horizontal synchronizing signals to be reproduced are different from each other, there occurs rolling of a reproduced image when the video signal is reproduced as it is in an intermittent slow reproduction mode. In order to prevent the rolling, in the intermittent slow reproduction mode, phase control is released, while the rotational speed of the cylinder motor is increased and decreased to coincide with the speeds in a still reproduction state and a normal reproduction state.
In the case of the transition from a state in which phase control is performed (normal reproduction mode) to a state in which phase control is released (intermittent slow reproduction mode) and the reverse transition, there occurs the following problem. More specifically, the problem is how to set the phase error signal to be supplied to the cylinder motor driving system when the mode is changed between the normal reproduction mode and the intermittent slow reproduction mode.
Conventionally, similarly to a control method at the time of starting a motor which is disclosed in Japanese Utility Model Publication No. 40650/1984 and Japanese Patent Laying-Open Gazettes Nos. 202358/1986 and 212179/1986, a signal at a predetermined level is applied to the cylinder motor driving system as a phase error signal during a period of releasing phase control of the cylinder motor.
However, according to the conventional method, discontinuing of the phase error signal occurs when the mode is changed between the normal reproduction mode and the intermittent slow reproduction mode and much time is required until the phase is locked after the mode is changed, so that color synchronization of a video circuit is released. More specifically, in the conventional structure in which a signal at a predetermined level is only applied as a phase error signal when phase control is released, the phase error signal becomes discontinuous if the transition from the phase controlled state to the phase control released state, so that the cylinder motor is irregularly rotated. On the other hand, in the case of the transition from the phase control released state to the phase controlled state, about two to three seconds are required until the phase is locked, so that color synchronization may be released and the reproduced image may be very unclear.